The goals of this project are to 1) develop radioimmunologic techniques to search for retrovirus expression in cells and tumors of human origin, 2) isolate recombinants between mouse leukemia viruses to provide genetic evidence for viral gene involved in leukemogenesis, 3) establish the genetic map of mammalian type-C viruses, 4) identify and characterize the transforming proteins of murine and feline sarcoma viruses and 5) determine the mechanisms of sarcoma virus-induced cell transformation.